WASHINGTON (CNN) -- The general election season opens with a neck-and-neck race between Barack Obama and John McCain, with more than one in five voters acknowledging that they might change their minds between now and November.

In the CNN/Opinion Research Corporation poll, the first conducted entirely after Obama became the presumptive Democratic nominee, he leads his Republican counterpart 49 percent to 46 percent among registered voters -- a statistical tie, given the question's 3-point margin of error.

McCain and Obama aren't alone. Independent candidate Ralph Nader and Republican-turned-Libertarian Bob Barr are vying with the two major-party candidates for independent voters.

But at this point, it looks unlikely either will play a spoiler role: When pollsters asked about a field of candidates that includes Nader and Barr, the margin between Obama and McCain was virtually unchanged, with the Illinois senator leading 47 percent to 43 percent. Nader pulls in 6 percent and Barr 2.

What would Hillary Clinton bring to a Democratic ticket? Answer: About 3 percentage points.

A hypothetical Obama-Clinton ticket would currently get 52 percent of the vote, compared with 46 percent for a hypothetical McCain-Romney ticket, according to the poll. If Clinton is not on the ticket, 60 percent of her Democratic supporters said they would vote for Obama, 17 percent would vote for McCain, and 22 percent would stay at home in November and not vote for anyone. Watch Obama plan his second presidential term »

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"That's just one estimate of the 'Clinton factor,' " said CNN polling director Keating Holland, "and it may not be an accurate predictor since it piles several hypotheticals on top of each other and asks people to guess their state of mind five months from now.

"Nonetheless, it does indicate that unmotivated Clinton supporters may be a bigger risk to Obama than defections from the Clinton camp to McCain."

The results are based on interviews with 1,035 adult Americans, including 921 registered voters, conducted by telephone Wednesday and Thursday.